The invention relates generally to instrumentation amplifiers which are defined as amplifiers having a differential input, both terminals of which display high impedance, and a single ended output. Such amplifiers are typically coupled to transducers to provide signal translation that acts to develop a single ended replica of the differential transducer signal at a level and amplitude suitable for subsequent signal processing. The amplifier has a high common mode signal rejection and power supply variation rejection.
In many applications the transducer is located remotely from the amplifier in a relatively inaccessible spot and a transmission line is used to couple the transducer to the amplifier input. It is also common practice to ground the transducer at its remote location. Since the amplifier ground includes one of the power supply connections, any potential difference between the local and remote grounds appears as a common mode potential and is rejected. However, in monolithic integrated circuit (IC) amplifiers there may be a problem. In conventional IC design, the circuit chip substrate is commonly connected to the negative power supply terminal. The transistors and other circuit elements fabricated into the substrate are PN junction isolated from the substrate. The isolated components therefore have to be reverse biased with respect to the substrate in order to make the isolation effective. In the typical NPN transistor the collectors are the junction isolated elements and their base and emitter electrodes are diffused therein. In this configuration the collectors must be positively biased with respect to the substrate and the emitters are typically returned to the substrate as the most negative circuit potential. Thus, the input transistor bases must be positive with respect to the substrate. In other words, the input transistor bases must be biased at a potential that lies within the span of the power supply potential.
In the case where the input common mode is to include potentials more negative than the negative power supply potential, it is common practice to add a second negative power supply whose span embraces the negative common mode potentials. The most common configuration for monolithic IC instrumentation amplifiers is to operate them from equal positive and negative power supplies with the common point representing the circuit ground which is also the nominal transducer operating level.
In many applications it is not practical or convenient to employ a dual power supply. For example, in vehicular or automotive applications a single battery having one terminal grounded to the vehicle frame is available. It is impractical to implement a dual supply. Also in this environment, where the transducer is frame grounded at one location and the amplifier grounded at a different point, it has been found that the common mode potentials can be on the order of several volts and of either polarity. Such potentials are dependent upon many uncontrolled factors and are therefore unpredictable. In a system having a negative battery ground at the amplifier, positive transducer common mode potentials are no problem and a conventional IC can cope with them. However, for negative common mode potentials conventional IC circuit designs will be inoperative.